This invention relates to a device and method for selecting optical pulses.
A multitude of applications, e.g. certain fields of laser machining, photo and display technology, biomedical screening techniques based on fluorescence spectroscopy, laser range finding, LIDAR, and optical analytics require the provision of very short optical pulses (down to the femtosecond range) at a low repetition rate (up to 100 MHz) Since conventional short-pulse laser systems, such as mode-coupled lasers, generate pulses at a high repetition rate, fast optical modulators (pulse pickers) are needed that can select and transmit individual pulses from the fast pulse trains, that is, achieve a frequency division. These singled out pulses could then be separately processed. Unlike mode-coupled laser systems, systems with a modulator provide the option to generate pulses of almost any shape, length, and repetition rate.
Pockels cells that work with non-linear voltage-dependent optical crystals are a known means of selecting pulses. Kerr cells with a field-dependent polarization are also known to be usable. It is a disadvantage that they require high voltages. The capacities for which charges have to be shifted accordingly are also very high. The minimum pulse width (and thus the maximum input pulse repetition rate) that can be achieved and the repetition rate of the switch (and thus the maximum output pulse repetition rate) are very limited. Pulse widths of several 10 ns (typically 30 ns) can be reached. The repetition rate is several MHz.
The use of acousto-optical modulators is also known for pulse selection. A standing ultrasonic field is generated by applying a RF voltage. This field then diverts the optical laser pulses. The generation of a standing field takes time. Pulses of at least several ns can be achieved in this way. The repetition rate is limited by the maximum mean output so that not much more than 10 MHz is reached.
Furthermore, the use of electro-optical modulators is also known for pulse selection. The simplest example of electro-optical modulators consists of a crystal, e.g. lithium niobate, in which the refractive index depends on the strength of the electrical field applied. Therefore the transparency of the crystal is a function of the applied field strength. Electro-optical crystals change their optical thickness instantaneously as a function of the strength of an applied electrical field. The effect depends on the polarization of the incident radiation. The optical path difference for two orthogonally polarized beams is 180°, when the so-called λ/2 radiation is applied. The polarization plane turns by 90° for incident linearly polarized light if the crystal is adjusted accordingly. A polarizer completely removes the light from the beam path. The intensity of the light passing through can be modulated by varying the voltage applied. The modulator can therefore be viewed as a phase retardation plate with electrically adjustable retardation. Optical pulses can therefore be selected by voltage variation.
Furthermore, the use of an integrated electro-optical modulator on waveguide basis is also known for pulse selection. The base element is a phase coupled Mach Zehnder amplitude modulator based on the ferroelectric crystal material, lithium niobate. Modulation is performed by electro-optical detuning of the waveguide interferometer due to an electric voltage applied to the electrode system.
The light modulators described have the disadvantage that they are relatively big and require much adjustment, and that the modulation frequency is not large enough for separating individual pulses.
U.S. Patent Publication No. 2005/0206989 A1 discloses a multi-bandgap modulator that can have a broader range of wavelengths due to separately addressable sections of different bandgaps, or can compensate chirp.
EP 1 065 765 A2 describes a laser with improved modulation across a wide range of wavelengths. The laser diode and the modulator are grown in one structure, i.e. onto a common substrate, and the modulator comprises at least a first and a second range in which modulation of the light of the laser diode is achieved by applying a reverse voltage.
U.S. Patent Publication No. 2003/0180054 A1 discloses an optical modulator for ultrafast pulses, wherein the driver circuit and modulator are integrated onto one substrate and the between the driver circuit and the optical modulator is smaller than one tenth of the wavelength of the modulation frequency. The maximum modulation frequency can be achieved by reducing the inductance of the electric lines between the driver circuit and the optical modulator.
However, it is a disadvantage of these modulators mentioned above that, depending on the intensity of the pulses in the pulse picker section—i.e. in the modulator—charge carriers can form that result in an undesirable transparency of the pulse picker section although no charge carriers are injected via the contacts. The optical modulator therefore has a relatively low breakdown resistance.
EP 1 210 754 B1 describes an electro-optical modulator with a large extinction ratio and an improved nonlinear extinction curve, wherein a semiconductor laser source, the electro-optical modulator, and a semiconductor amplifier are integrated onto one substrate. Therefore, this device cannot be used for modulating laser pulses generated externally because these cannot be coupled in. In addition, the device can only be used for modulating low outputs.
U.S. Pat. No. 5,798,856 discloses an optical laser pulse generator for glass fiber communication that reduces a wave packet broadening due to dispersion.